Reduce number of calculations in FSTCompiler (#13788)

* Simplify FST return

* Reduce number of calculations in FSTCompiler

lucene/core/src/java/org/apache/lucene/util/fst/FSTCompiler.java

@@ -817,7 +817,8 @@ private void freezeTail(int prefixLenPlus1) throws IOException {
  for (int idx = lastInput.length(); idx >= downTo; idx--) {
 
  final UnCompiledNode<T> node = frontier[idx];
- final UnCompiledNode<T> parent = frontier[idx - 1];
+ final int prevIdx = idx - 1;
+ final UnCompiledNode<T> parent = frontier[prevIdx];
 
  final T nextFinalOutput = node.output;
 
@@ -833,7 +834,7 @@ private void freezeTail(int prefixLenPlus1) throws IOException {
  // this node makes it and we now compile it. first,
  // compile any targets that were previously
  // undecided:
- parent.replaceLast(lastInput.intAt(idx - 1), compileNode(node), nextFinalOutput, isFinal);
+ parent.replaceLast(lastInput.intAt(prevIdx), compileNode(node), nextFinalOutput, isFinal);
  }
  }